[FieldTrip] interactions between two factors

Marco Buiatti marco.buiatti at gmail.com
Fri May 25 15:58:32 CEST 2012

Dear FieldTrippers,

I am analysing an EEG study with 2x4 factors: one varies between 4
parametrically varying levels (1 to 4), the second between two levels.

I have three questions concerning the use of Fieldtrip cluster-based
non parametric statistical analysis in this case:

1) How to compute the interaction between the two factors. Let's start
from the simplest case of a 2x2 design, factors varying between values
A1 and A2 for the first factor, B1 and B2 for the second. Please tell
me if it is correct to compute the interaction by:
- computing the difference diffA=ERP(A1)-ERP(A2) separately in
condition B1 and B2, for every subject
- performing a within-subjects statistical analysis between diffA in
condition B1 and diffA in condition B2 (function

2) Now consider that factor A varies parametrically between values 1
to 4. For the main effect of this factor, I have used the Fieldtrip
function statfun_depsamplesregrT.m and I'm satisfied with it. Is it
correct to compute the interaction by
- computing the regression
regrA=regression(ERP(A1),ERP(A2),ERP(A3),ERP(A4)) (computed as inside
function statfun_depsamplesregrT.m) separately in condition B1 and B2,
for every subject
- performing a within-subjects statistical analysis between regrA in
condition B1 and regrA in condition B2 (function

3) Since BEFORE looking at the data (this is to prevent Eric's
contestation...) I expect a dipolar topography for the regression
(data are in average reference), I would like to combine into a joint
cluster negative and positive clusters. I have tried by changing
statfun_depsamplesregrT.m by just taking the absolute value of the
regression, but I get weird results (a huge, non significant cluster).
Is it possible that since values are now all positive, I should use a
different statistical test at the single bin level? Any other

Thanks in advance for your help,


Marco Buiatti, PhD

CEA/DSV/I2BM / NeuroSpin
INSERM U992 - Cognitive Neuroimaging Unit
Bât 145 - Point Courrier 156
Gif sur Yvette F-91191  FRANCE
Ph:  +33(0)
Fax: +33(0)
E-mail: marco.buiatti at gmail.com


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